1. Field of the Invention
The invention relates to a method of producing metal strip in a plant, comprising a continuous casting plant, a first furnace adjoining in conveying direction, a second furnace adjoining the first furnace in conveying direction and a rolling mill adjoining the second furnace in conveying direction. In addition, the invention relates to a plant for producing a metal slab or a metal strip.
2. Description of the Prior Art
Plants for producing metal strips in the stated manner are known in the prior art.
It is known from EP 1 960 131 B1 to activate or deactivate a holding furnace and an induction furnace for heating slabs in dependence on a selected mode of operation, namely in one instance in continuous production of a metal strip and in one instance in discontinuous production of the strip.
EP 1 963 034 B1 provides, for heating a slab, an induction furnace, the working frequency of which is selected to be low so that the heating effect is concentrated on the slab core.
DE 10 2008 055 650 A1 describes a method for minimising the energy requirement and the CO2 emission in thin slab plants, wherein use is made of computation models by which, in particular, the water quantity needed for cooling and distribution thereof in the plant as well as casting speed are determined.
The combined use of a holding furnace and an induction furnace is also known from EP 1 469 954 B2 and from U.S. Pat. No. 7,942,191 B2.
In the case of production of hot strip in a so-named CSP plant initially the thin slab is cast in a continuous casting plant, then heated in a roller hearth furnace to the desired furnace temperature and directly thereafter rolled down in the finishing train (rolling mill) to the finished strip thickness. Heat energy (for example in the form of gas) is needed for reheating the thin slab in the roller hearth furnace and current for reshaping is needed for the thickness reduction in the finishing train. The necessary furnace temperature in that case substantially depends on the final thickness and strip width to be rolled as well as on the strip material.
Strips with lower final thickness or high loading in the finishing train in that case predominantly determine the maximum furnace temperature level (for example 1,150° C.). However, these extreme strips often represent only a smaller proportion of production. Different strips are rolled within a rolling program or within a day. A number of the strips does not require the high entry temperature, i.e. these strips are overheated. Heating energy could be saved here. However, a roller hearth furnace cannot be set as desired to each strip and the finishing train entry temperatures (T-FM; defined as mean slab temperature after heating or behind the last furnace in front of the finishing train) are changed individually. Due to the inertia of the roller hearth furnace the furnace temperature accordingly remains at substantially the same high level. The typical conceptual construction of such a previously known CSP plant is evident from FIG. 1; the course of the mean temperature over the length of the plant, i.e. from the continuous casting plant to behind the finishing train, is apparent from FIG. 2.
In FIG. 1 a plant is diagrammatically depicted which comprises a continuous casting plant in which a slab 1 is cast. The slab 1 passes into a roller hearth furnace 3, where it is heated to an entry temperature TFM. Here the slab has, for example, a thickness of 60 mm and moves in conveying direction F at a speed of 5 m/min. The roller hearth furnace is, for example, 240 m long. The slab 1 is thereafter conducted at the temperature TFM behind the furnace into the rolling mill 5 (finishing train) and rolled down to the strip of desired thickness, for example to a value of 2.4 mm. The strip subsequently passes onto a cooling path 10.
The corresponding course of temperature is evident from FIG. 2. It can be seen here that the entry temperature TFM lies at 1,150° C.
The energy expended in the roller hearth furnace and for the reshaping in the finishing train (from, for example, 60 mm slab thickness after the continuous casting plant to 2.4 mm final strip thickness) as well as the CO2 emissions and energy costs can be given as follows for an example with an entry temperature TFM of 1,150° C. (the energy consumption of auxiliary units such as scale washing pumps, pumps for roll cooling, etc., were not taken into consideration):
The roller hearth furnace with water-cooled rollers needs a heating energy of 178.1 kWh/t. The heating costs are 5.34 Euro/t and the CO2 emissions 41.0 kg of CO2/t. A reshaping energy of 47.8 kWh/t with power costs of 3.35 Euro/t and a CO2 emission of 26.8 kg of CO2/t are needed in the finishing train for the reshaping. In total, an energy consumption of 225.9 kWh/t is incurred with costs of 8.69 Euro/t and a CO2 emission of 67.7 kg of CO2/t.